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Diagrams of the tetragonal and triclinic I-centered cells showing the effects of the partial order-disorder phase transition. At 33 K the Fe–NO group retains the disorder with respect to crystallographic inversion but the disorder of the NO group due to fourfold symmetry has been removed. In the tetragonal phase the porphyrin plane is perpendicular to the c axis, but is tipped by 9.3° from the perpendicular to the c axis in the triclinic phase.
